Abnormality in Wnt signaling is causatively associated with oxidative stress-induced intestinal tumorigenesis in MUTYH-null mice

Abstract

MUTYH is a DNA glycosylase that excises adenine paired with 8-oxoguanine to prevent mutagenesis in mammals. Biallelic germline mutations of MUTYH have been found in patients predisposed to a recessive form of familial adenomatous polyposis (MAP: MUTYH-associated polyposis). We previously reported that Mutyh-deficient mice showed a high susceptibility to spontaneous and oxidative stress-induced intestinal adenoma/carcinoma. Here, we performed mutation analysis of the tumor-associated genes including Apc, Ctnnb1, Kras and Trp53 in the intestinal tumors of Mutyh-deficient mice. In the 62 tumors, we identified 25 mutations in Apc of 18 tumors and 36 mutations in Ctnnb1 of 36 tumors. Altogether, 54 out of the 62 tumors (87.1%) had a mutation in either Apc or Ctnnb1; no tumor displayed mutations simultaneously in the both genes. Similar to MAP, 60 out of 61 mutations (98.3%) were identified as G:C to T:A transversions of which 85% occurred at either AGAA or TGAA sequences. Immunohistochemical analyses revealed the accumulation of β-catenin in the nuclei of tumors. No mutation was found in either Kras or Trp53 in the tumors. These results indicate that the uncontrolled activation of Wnt signaling pathway is causatively associated with oxidative stress-induced intestinal tumorigenesis in the Mutyh-deficient mice.

Keywords: DNA repair; MAP; Wnt signaling pathway; mutagenesis; oxidative DNA damage.

Figure 1

H.E. staining of the KBrO₃-induced small intestinal tumors developed in Mutyh-deficient and wild type mouse. (A) One of the typical tumors from Mutyh-deficient mouse; classified as category 4. (B) An exceptional case classified as category 3 developed in wild-type mouse. Magnifications are indicated in lower right of the photos.

Figure 2

Immunohistochemistry for β-catenin. Many nuclear steins are observed in tumor cells but not in normal cells. The junctions of epithelial cells are also well stained. Magnifications are indicated in lower right of the photos.